1. Field of the Invention
The present invention relates to an image reading device performing image reading with high precision.
2. Description of the Prior Arts
High precision of an output image from a printer or the like has been recently developed, and therefore, a demand for high-speed and high-precision to an image reading device has risen. However, in an image reading device used for a normal digital copying machine or the like, only ones having its reading resolution (sampling density) of about 400 to 600 dpi have been put on a practical use due to a limitation in a pixel number, sensitivity, driving speed or the like of an image sensor used therefor. There has been a problem of generating a sampling moire when, for example, a printing document having a mesh screen structure is read by a reading device having only a reading resolution of 400 to 600 dpi. Further, a problem has occurred that reproducibility of a fine character in a map document or the like is insufficient.
It is effective for controlling the sampling moire that reading MTF in the vicinity of the Nyquist rate is reduced by optical parts such as a birefringent plate or the like or image processing parts such as a filtering. However, these methods involve a blur in the reading image to entail a problem of deteriorating reproducibility of a character.
An image reading device having a high sampling density that does not deteriorate the character reproducibility even if the reading MTF is reduced at the Nyquist rate is required, in order to obtain both of restraint of the sampling moire and the character reproducibility. However, it is very difficult to manufacture the image sensor having more than twice as many pixels as a conventional one in order to deal with this, whereby it is difficult to obtain a high-speed and highly precise reading device.
Therefore, the image reading device is branched to two types, i.e., a high-speed reading device for a digital copying machine and a reading device with low speed and high precision used by connecting with a PC. However, it is desired to obtain one reading device capable of being performing high-speed reading and highly precise reading.
On the other hand, there has been proposed a technique for attempting to establish high resolution with a sensor of less pixels by using an image shift method in which a sampling phase is shifted by xc2xd pixels (Japanese Unexamined Patent Application No. Sho 51-25914 or the like) . In particular, there have been many proposals about the usage for a video camera using an area sensor (Japanese Unexamined Patent Application No. Hei 6-261236). This method performs sampling with a density double that of the sensor pitch, so that the size of the photosensitive pixel becomes approximately double that of the sampling pitch. Accordingly, the Nyquist resolution can theoretically be reduced to restrain the sampling moire, so that this method is suited for highly precise reading. As a part that implements such an image shift method, there has been known a device in which an optical flat plate is disposed in an optical path, a device using an optical element that prevents a camera from moving upon pressing the button, called as a variable-angle prism, a device in which a sensor itself is slightly moved and the like.
However, the image shift method according to a conventional technique performs the image shift for every exposing period of a reading sensor. When such a mechanism is applied to high-speed reading by a linear sensor, the exposing period per line is short, so that the image shift mechanism has to be driven with high speed of several thousands Hz. Further, there is a drawback that the vibration due to the driving of the image shift mechanism causes a bad influence to a reading image quality. Therefore, the conventional technique entails a problem of not dealing with high-speed reading.
Moreover, the image shift mechanism cannot be perfectly halted in order to perform high-speed driving of the image shift mechanism simultaneous with the slow-scan, so that the image cannot be rendered stationary, to thereby entail a problem of not being capable of strictly forming a new sampling point between pixels. Japanese Unexamined Patent Application No. Hei 7-283915 discloses a combination of the image shift method and digital interpolation of a sampling point with respect to this problem. However, the number of the substantial sampling point is reduced, whereby there is a problem of not sufficiently utilizing an inherent performance of the image shift method.
On the other hand, when the image shift method is executed by using an area sensor, its driving frequency is normally low-speed such as several ten Hz, whereby a minute image shift can actually be implemented. However, the number of pixels of the area sensor generally used is several million (1000xc3x971000 to 2000xc3x973000 pixels) at most, thereby entailing a problem that it is insufficient for highly precise reading of a reflective document.
In order to solve the problems, the present invention provides an image reading device capable of performing a high-speed and highly precise reading without generating a sampling moire by using an image shift method.
The image reading device of the present invention has a reading part that reads an optical image of an object to be read, a moving part that moves in a slow-scanning direction a reading position of the object, a scan controlling part that scans the object plural times by controlling the moving part, a reading position changing part that changes in a fast-scanning direction the reading position of the object for every scan by the scan controlling part, and a moving speed changing part that changes the moving speed of the reading position in accordance with the number of scanning times.
With the above configuration, when an optical image of the object to be read is read while moving the reading position of the object to be read by the reading part in the slow-scanning direction by the moving part, the moving part is controlled by the scan controlling part so as to scan the object to be read plural times as well as the reading position of the object to be read by the reading part is changed by the reading position changing part in the fast-scanning direction for every scan by the scan controlling part, and further, the moving speed by the moving part is changed by the moving speed changing part in accordance with the scanning times by the scan controlling part. Therefore, it is possible to perform highly precise reading with a several-fold sampling density only by adding a simple mechanism, so that it is possible to eliminate a phenomenon inherent to digital reading such as moire. Further, the scan is performed in the slow-scanning direction with a scanning speed in accordance with the scanning times when performing a reading scan plural times, whereby the sampling density in the slow-scanning direction can be changed, as well as the sampling density in the fast-scanning direction can be changed by performing the image shift for every reading scan. Accordingly, it is unnecessary to execute the image shift in a short time, to thereby be capable of performing high-speed and highly precise reading.